1. Field of the Invention
The present invention relates to semiconductor processing and more particularly to systems for monitoring semiconductor processing steps.
2. Description of the Related Art
The manufacture of high density integrated circuit devices typically proceeds by repeating a sequence of processing steps which includes depositing a layer of material over a substrate, forming an etching mask on the surface of the deposited material, etching the deposited material to define a structure over the substrate, and then removing the etching mask. This sequence is repeated many times in the manufacture of a typical integrated circuit device. Due to the economics of the manufacture of integrated circuit devices, it is very desirable to minimize the time spent in individual processing steps and in the total time consumed for this sequence of processing steps. On the other hand, it is very desirable to complete each processing step in such a manner as to ensure that there are high yields from the manufacturing process. To this end, it has become increasingly common to incorporate systems for monitoring the progress of processing steps as an essential component of integrated circuit manufacturing equipment.
Etching processes are particularly amenable to the use of process monitoring techniques because of the often critical nature of the etching process and because of the presence of detectable byproducts of the etching process. A typical etching process is performed to remove the portion of a layer of deposited material that is exposed through the openings in an etching mask. Exposed material is generally removed using an anisotropic process to etch holes having substantially vertical walls aligned with the mask openings, with the etched holes extending all the way through the layer of deposited material. Material removed by the etching process is typically present in the etching chamber either within a gaseous end product of the etching process or as particulates. The progress of etching processes should result in changes in the composition of gases within the etching chamber during different portions of the etching process. For example, the fraction of chamber gases incorporating materials derived from the material being etched should increase as etching begins and should drop when the etching process is complete. Monitoring the presence within the etching chamber of reacted gas compounds incorporating the etched material can therefore provide an indication of the end of the etching process.
Different monitoring techniques are used for detecting the presence of gaseous byproducts of etching processes within the etching chamber or for otherwise monitoring the progress of a processing step. One of the more common process monitoring techniques is used to monitor the progress of plasma etching processes. Plasma etching processes can highly excite the elemental etched material (i.e., particulates) or the compounds incorporating reacted etched material so that these materials fluoresce or otherwise emit light at wavelengths that are characteristic of the etched material. By monitoring the intensity of fluorescence or characteristic emissions at these characteristic wavelengths, the etching process can be monitored. A drop in the intensity of fluorescence might, for example, indicate the completion of an etching process and so might be used as an endpoint detection tool.
Other process monitoring techniques are known. For example, instead of monitoring an emission that is characteristic of an etched material, light can be passed through the etch chamber to measure optical absorption on lines that are characteristic of the etched material or the byproducts of the etching process. An increase in absorption may be characteristic of the onset of etching and a decrease in absorption may be characteristic of the completion of etching. Still other techniques are know or could be used. Ellipsometric measurements are used to detect the thickness of films deposited onto workpieces and might be used to monitor the etching of thin, relatively uniform films. Reflectance measurements including reflectance spectroscopy can be used to detect the presence or absence of a film on the surface of a workpiece and so might be used for detecting the deposition of a film or removal of a film in an etching process. Each of these techniques could be used in an endpoint detection system.
A common feature of these process monitoring tools is the use of optical techniques to remotely sense the presence or absence of materials in the etching chamber. For at least some types of processing equipment, it is necessary to provide an optical path or optical viewing port to gain access to the processing environment to make such optical measurement techniques. Most processing systems are enclosed within chamber walls so that the processing chamber can be evacuated and provided with different gas environments as desired. While some of the processing chamber walls are formed from transparent materials such as quartz, it may be necessary to provide transparent windows through the chamber walls when optical monitoring is to be performed through walls that are not transparent. It may also be necessary to provide an optical path or viewing port through structures that are provided around the processing chamber. For example, in some plasma etching systems, the etching chamber is contained within electrical coils that are used for applying either a radio frequency (RF) electric field or a magnetic field to the interior of the etching chamber. An example of such a system is the Hitachi 308-ATE ECR Metal Etcher, which provides coils formed of hollow copper tubing around the quartz walls of the etching chamber. The copper coils are provided around the etching chamber to provide an RF field to the interior of the chamber, and completely surround parts of the chamber walls. To implement an endpoint detection system for this metal etcher, it is most convenient to provide an optical fiber through the coils to a fixed position adjacent the quartz chamber wall so that the fiber images at least a portion of the etching chamber. Sensing operations can then be performed through the optical fiber to facilitate operation of an endpoint detection system.